The invention relates to a magnetic field detector unaffected by external parameters such as temperature and pressure, for example.
In a ring interferometer, two beams traverse an identical optical path in opposed directions and interfere upon emerging from this path. Provided that a disturbance of this path has identical characteristics for both directions of propagation and does not vary during the transit period of the light in the interferometer, the two beams are affected identically and their relative phase remains unchanged. The disturbances of this kind are referred to as "reciprocal". Because the transit period in an interferometer is commonly very short, the variations of a disturbance are commonly negligible during this period, unless the same is introduced intentionally.
However, there are "non-reciprocal" disturbances which have a different amplitude in the two directions of propagation, these consisting of physical actions, which, by establishing its overall orientation, destroy the symmetry of the space or environment.
Two known effects have this property:
the Sagnac effect, or relativistic inertial effect, in which the rotation of the interferometer with respect to a Galilean datum destroys the symmetry of the propagation periods;
the Faraday effect or colinear magnetooptical effect, in which a magnetic field generates a preferential orientation of the spin of the electrons of the optical material.
A prior art device described in the European patent application made by the applicants and published on Jan. 28, 1981 under the number 0 023 180, relates to a current measuring device comprising an optical fibre wound around a conductor in which flows the current I which is to be measured, this optical fibre comprising one or more turns, the two extremities of this optical fibre each receiving an optical radiation coming from a laser, for example; these two waves flows in the fibre in opposed directions. The current flowing in the conductor induces a magnetic field in the same direction as the direction of propagation of one of the waves and in the opposed direction to that of the other. The two waves emerging from the fibre have a phase displacement .DELTA..PHI. which depends on the Verdet constant characteristic of the Faraday effect of the propagation medium, on the intensity I of the current flowing in the conductor, possibly on the number N of conductors if the optical fibre surrounds several conductor branches wherein flows the same current I, and on the number M of turns of the optical fibre encircling the conductor. (The Verdet constant, a constant of proportionality in the equation of the Faraday effect, is equal to the angle of rotation of plane-polarized light in a magnetized substance divided by the product of the length of the light path in the substance and the strength of the magnetic field.)
To demonstrate the phase displacement between the two waves, this measuring device utilises an interferometer structure of the "Sagnac" type, the two counterrotating waves emerging from the extremities of the fibre being recombined and the corresponding signal being detected by a photodetector. These two waves thus undergo, in the same manner, the reciprocal effects which within the medium induce variations varying in the same direction in the conditions of propagation, and by non-reciprocal Faraday effect undergo variations in the opposed direction. These variations in opposed direction are liable to be detected by an interferometer method.
Compared to this prior art device, the device of the invention offers different advantages, such as its considerable simplicity and small number of components. Furthermore, no alignment of any kind is needed, unless it is that of the source and of the fibre, which forms part of the prior art. Moreover, this device has considerable geometrical flexibility in particular regarding the waveguide length and the geometrical arrangement imparted to this wave guide.
Only "non-reciprocal" disturbances have an effect on the signal detected, in the device of the invention. The dimensional variations, such as flowage, thermal expansion, pressure variation or refraction index variations, for their part have no effect on the signal detected. An instrument for measuring "non-reciprocal" effects, which offers perfect stability, is thus available in principle.
In practice, so that the reciprocal disturbances have absolutely no effect, the two beams of the interferometer should travel along precisely the same trajectory. More specifically, the two waves should be two identical solutions of the wave equation of the interferometer, the sign of the "time" parameter being reversed.
This condition is never rigorously fulfilled if the interferometer is constructed for free propagation, which is the case of the application of discrete optical elements:
the wave equation provides a "continuum" of solutions and at least misalignment of the optical system leads to obtaining different solutions, that is wave fronts which are not superposed,
even for solutions which are identical when considering waves of infinite extension, plane waves for example, the distribution of intensity which is perforce limited in practice, actually differs if only because of the diffraction, and disrupts the reciprocity.
A solution of the single-mode type consequently consists in a device produced as a wave guide structure from end to end.